This invention relates to submergible electric pump motors and is particularly concerned with improvements in high temperature oil-filled motors.
For many years submergible pump units have been employed in the pumping of oil or water from wells. Representative submergible pump units are disclosed, for example, in U.S. Pat. Nos. 1,951,919; 1,970,484; 2,001,649; 2,236,887; 2,251,816; 2,270,666; 2,315,917; 3,672,795; and 4,275,319, all assigned to the assignee of the present invention. Typically, a submergible pump unit comprises an electric motor and a centrifugal pump suspended colinearly in a well by tubing or cable. The motor is filled with an oil that serves to lubricate moving parts, insulate electrical parts, cool the motor, and exclude the ambient well fluid from the interior of the motor. A protector containing oil accommodates the thermal expansion and contraction of the oil that results from the running and non-running cycle of the motor.
Submergible electric pump motors are expensive. When they fail in service, the pump unit must be pulled from the well, which is an expensive procedure. Accordingly, sophisticated insulation systems are employed in the pump motors to minimize electrical breakdowns. For example, the stator insulation system of one type of submergible motor sold by the assignee of the present invention includes insulating sleeves and insulating laminations at opposite ends of the stator, insulating slot liner tubing for the stator windings, multiple layers of insulation on the magnet wire of the windings, insulating tape on the end turns of the windings, insulating sleeves on the connections to the end turns, and an insulating varnish that impregnates the entire internal stator structure. The improved insulation system disclosed in U.S. Pat. No. 4,275,319 (referred to earlier) greatly increases the mean time to failure of prior insulation systems. Nevertheless, in the harsh environment of high temperature wells, even this improved insulation system has not provided the desired useful life of submergible motors.
It has been standard practice in oil-filled submergible motors to include a varnish in the stator insulation system. The invention of the aforesaid U.S. Pat. No. 4,275,319 relies upon an improved hydrolytically stable varnish to extend the life of the motor. The varnish impregnates substantially the entire stator system and serves as a moisture barrier to protect the magnet wire insulation from hydrolytic attack. In a conventional manner, the varnish also serves as a secondary electrical insulation for the electrical conductors of the stator, supports the weight of the stator windings, and encapsulates the end turns of the stator windings so that they are substantially immobilized.
The use of varnish in the stator insulation system is a mixed blessing, however. The varnish impregnation process is an expensive, labor and capital intensive operation. The varnish tends to degrade in use, causing contamination of the oil that fills the motor. Moreover, repeated thermal expansion-contraction cycling that occurs during the operation and shut-down cycles of the motor cracks the varnish and ultimately leads to electrical failure of the motor. Other problems associated with the use of varnish will be considered later.